The invention is directed to an improved electrolytic cell for the separation of metals from electrodissociatable compounds in the molten state. It is particularly useful for the separation of alkali metals.
The metals most frequently made by electrolysis of electrodissociatable compounds in the molten state are the alkali metals, particularly sodium and lithium.
A considerable proportion of the elemental alkali metals which are manufactured for commerce is produced by the electrolysis of molten halogen salts of the metals, especially low melting mixtures of such salts with other salts which are inert. For example, sodium metal can be produced by electrolysis of a molten binary mixture comprising calcium chloride and sodium chloride or a ternary mixture such as sodium chloride, calcium chloride and barium chloride. On the other hand, lithium metal is produced by electrolysis of a molten binary mixture comprising potassium chloride and lithium chloride.
The type of electrolytic cell most widely used for the above-described operations is the Downs cell, which is described in U.S. Pat. No. 1,501,756 to J. C. Downs. The Downs-type electrolytic cell basically is comprised of a refractory-lined steel shell for holding the molten salt electrolyte, a submerged cylindrical graphite anode surrounded by a cylindrical steel cathode and a perforated steel diaphragm positioned in the annular space between the electrodes to separate the anode and cathode products. To collect product halogen gas from the anode, the cell is provided with collector means such as an inverted cone which fits over the anode below the surface of the molten bath. Halogen gas (usually chlorine) passes upwardly through the cone and, via appropriate manifold components, from the cell. Similarly, the cathode is also provided with collector means such as an inverted inclined trough which fits over the cathode below the surface of the molten bath. Molten alkali metal rises from the cathode toward the surface of the molten bath, is collected along the inclined surface of the trough and is passed to a vertical riser/cooler in which the molten metal is partially cooled before it is passed to a product receiver.
Despite the current technical and economic superiority of the Downs cell for making alkali metals, particularly sodium and lithium, the cell nevertheless has several disadvantages which are becoming even more highly significant as additional emphasis is placed on energy conservation and the quality of working environment for operating personnel.
For example, in the manufacture of sodium, it is necessary to use a molten salt bath temperature of about 500.degree.-600.degree. C in order to maintain the electrolyte components in the molten state. At this temperature (high with respect to the melting point of sodium) significant amounts of electrolyte salts and alkaline earth metals dissolve in the product sodium and tend to plug the cell riser/cooler. Thus, for reasons of product purity as well as safety of operating personnel, the riser/coolers of Downs cells are equipped with an agitation device of "tickler" by which the salts and extraneous metals which are precipitated therein can be prevented from plugging the riser pipe. Such devices are well known in the art and are described inter alia in U.S. Pat. Nos. 2,770,364, 2,770,592, 3,037,927 and 3,463,721. In addition, the heat produced by the operation of a battery of such electrolytic cells coupled with the necessity of conducting the operation in a closed building present problems of heat discomfort for operating personnel despite the use of extensive ventilation facilities. The waste heat requiring such extensive ventilation is generated by the passage of direct current through the cells and represents a large energy loss in addition to the energy required for the operation of the ventilation system.
Therefore, from the standpoints of energy consumption, product quality and the comfort of operating personnel, it is immensely desirable to have an electrolytic process and cell which is operable at substantially lower temperatures at the same or higher efficiencies.
A most promising route by which these disadvantages of the prior art can be overcome is to employ an electrolytic process in which a solid electrolyte material, which, under the influence of an electrical potential, is permeable to the flow of selected cations, but impermeable to the flow of other species, i.e., fluids, anions and other cations, to separate the anode and cathode compartments of the cell. A basic method for carrying out the electrowinning of alkali metals in this manner is disclosed in U.S. Pat. Nos. 3,404,036 and 3,488,271 to Kummer et al in which a flat plate of sodium beta alumina is used as the solid electrolyte material. A similar method is disclosed in U.S. Pat. No. 3,607,684 to Kuhn in which sheets of beta alumina are used as a diaphragm to separate the anode and cathode compartments of the electrolytic cell.
Though the cells of the prior art, which have employed solid electrolyte material as a separator between the cathode and anode, are effective in carrying out the electrolytic separation of metals from molten salts thereof, such cells have remained largely undeveloped and lack the configuration necessary to obtain efficient continuous operation on a commerical basis. In particular, the cells of the prior art have not been of such design as to provide for safe continuous cell operation in the event of breakage of the fragile solid electrolyte material, nor do such prior art cells permit efficient use of electrical energy and factory floor space by providing an acceptable ratio of solid electrolyte surface area to cell volume.